We aim to understand the toxicifies associated with the acellular hemoglobin (Hb) in the circulation by applying in vivo microvascular tools and methods. Our results will expand the knowledge and therapeutic alternatives to hemolyfic pathologies, blood transfusions and Hb-based oxygen carriers (HBOCs). Toxicity will be analyzed in terms of microcirculatory phenomena measured by quantifying changes in funcfional capillary density (FCD) in a hamster window chamber model. FCD is a general functional biomarker that integrates systemic, cardiovascular, cellular, and biochemical changes resulfing from Hb-based transfusions. We will use Hbs generated by recombinant technology and chemical modificafions that emphasize each of the properties that leads to a